I. Field
The present invention relates generally to data communication, and more specifically to techniques for performing spatial processing for wideband multiple-input single-output (MISO) and multiple-input multiple-output (MIMO) communication systems.
II. Background
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission and is denoted as an (NT, NR) system. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, where NS≦min {NT, NR}. NS spatial channels may be formed by the NS independent channels of the MIMO channel and used for data transmission.
For a time dispersive MIMO channel, a signal sent from a given transmit antenna may reach a given receive antenna via multiple signal paths (i.e., propagation paths). These signal paths may include a line-of-sight path and/or reflected paths, which are created when the transmitted signal reflects off reflection sources (e.g., buildings, obstructions, and so on) and arrives at the receive antenna via different signal paths than the line-of-sight path. The received signal at the receive antenna may thus include multiple instances (i.e., multipath components) of the signal sent from the transmit antenna. The delay spread L of the MIMO channel is the time difference between the earliest and latest arriving multipath components (of some certain minimum energy) for all of the transmit-receive antenna pairs in the MIMO channel.
Time dispersion in the MIMO channel causes frequency selective fading, which is characterized by a frequency response that varies across the system bandwidth (i.e., different channel gains for different frequencies). The multipath components are associated with different complex channel gains and may add constructively or destructively at the receiver. Time dispersion and frequency selective fading are more problematic for a wideband MIMO system with a wide system bandwidth.
Various techniques may be used to combat frequency selectivity in a wideband MIMO channel. For example, a multi-carrier modulation technique such as orthogonal frequency division multiplexing (OFDM) may be used to partition the system bandwidth into multiple (NF) orthogonal frequency subbands. The wideband MIMO channel may then be viewed as being composed of NF flat-fading narrowband MIMO channels, each of which may be decomposed into NS spatial channels. Data may then be transmitted on the NS spatial channels of each of the NF subbands.
For a MIMO system that utilizes OFDM (i.e., a MIMO-OFDM system), the wideband MIMO channel can be characterized with (1) a complex channel gain for each of the NF subbands of each of the NT·NR transmit/receive antenna pairs (i.e., NF·NT·NR channel gains in all) and (2) the noise floor at the receiver. The channel gains and receiver noise floor may then be used to select the data rate(s) for data transmission on the NS spatial channels of each of the NF subbands. The channel gains may also be used for spatial processing at the receiver and possibly the transmitter in order to transmit data on the NS spatial channels of each of the NF subbands. Thus, for the MIMO-OFDM system, frequency selectivity can be combated by treating the wideband MIMO channel as NF flat-fading narrowband MIMO channels and performing spatial processing separately for each of the narrowband MIMO channels. However, this frequency-dependent spatial processing can greatly increase computation complexity at the transmitter and receiver. Moreover, the receiver may need to provide a large amount of feedback information (e.g., the channel gains) to the transmitter to support frequency-dependent spatial processing.
There is therefore a need in the art for techniques to more efficiently perform spatial processing in a wideband MIMO system.